The change in its advisory came hours after the EU's health commissioner, Androulla Vassiliou, urged Europeans to postpone non-essential travel to Mexico and the US.

The death toll from the virus in Mexico has now risen to 150 and there are confirmed cases in Spain and Scotland.

The WHO's emergency committee raised the pandemic threat level from phase three to phase four (out of a possible six) at an emergency meeting last night. Phase four is the highest level the WHO has ever set and it is the first flu pandemic alert for 40 years.

Cases have been reported in Mexico City, together with the states of Oaxaca, San Luis Potosi, Mexicali and Baja California. Travellers should consult a doctor immediately if they show signs of flu-like symptoms.

The Mexican Secretariat of Health has advised people to avoid large crowds, shaking hands, kissing people as a greeting, or using the subway. Maintaining a distance of at least six feet from other persons and frequent hand washing may decrease the risk of exposure."

Around 315,000 British nationals visit Mexico every year, and a spokesman for the Mexican Tourism Board in London said that they had been fielding calls all morning from holidaymakers scheduled to go to or planning to visit Mexico in the near future. He said that they were not planning to cancel their trips, but looking for advice on precautions they could take.

Q: Will my insurance cover me if I cancel my holiday/flight?

In light of the FCO's amended advice, travellers due to fly out to Mexico on holiday will now be entitled to claim refunds.

The Association of British Insurers advises anyone considering cancelling pre-booked travel arrangements to talk to their tour operator/travel agent/air carrier to see if alternative arrangements can be made. Travel insurance policies can normally be transferred to cover any new destination.

Q: Will my insurance cover me if I get ill?

Your travel insurance will cover medical expenses, but check with your provider. The advice is that if you show any symptoms of sickness while on holiday, you should phone the 24-hour medical emergency service provided by your insurers.

The minimum medical protection recommended for Europe is £1m, and £2m for the US where hospital costs are higher. However, many quality policies automatically cover up to £5m.

Q: What precautions should I take?

According to a BAA spokesperson: "The Health Protection Agency (HPA) is closely monitoring the situation and the UK Government is reviewing any threat it poses to the UK. Passengers returning from a visit to an affected area are advised to monitor their health closely for seven days. If during this period they develop a feverish illness accompanied by one or more of a cough, sore throat, headache or muscle aches, they should stay at home and contact their GP by phone or seek advice from NHS Direct (0845 4647)."

Q: What will happen to me at the airport?

The Health Protection Agency has no plans to screen passengers on arrival at UK airports from Mexico, as they did during the Sars epedemic in 2003. "That would be a decision for the Government and the Foreign and Commonwealth Office," said a spokeswoman. "If a passenger feels ill in transit, there are measures in place for the cabin crew and for the captain to alert the port health authority prior to landing so that medical assistance can be provided on the ground."

AP news agency reported that passengers arriving at Manchester Airport today from Cancun faced no questions about symptoms of the illness or screening either in Mexico or when they arrived back in the UK.

The Port Health Authority has asked airlines to make it aware of any suspected cases.

Advice from a travel medical expert

Flu occurs in many animal species as well as in humans, such as horses, pigs and birds, but flu strains normally circulate only among members of their habitual host species. Occasionally a new strain emerges that is capable of making the leap from one species to another. This is what has now happened with swine flu: the new strain appears to have "hybrid" characteristics of pig, avian and human flu, with the added ability of now being able to circulate easily between humans.

How is it spread?

It spreads in exactly the same way as "normal" seasonal flu - by inhalation and via contaminated objects (for example, by touching them and then bringing your hand to your mouth).

How serious is it?

The cases in Mexico have had a much higher fatality rate than ordinary seasonal flu, with young adults more seriously affected than the usual risk groups for seasonal flu, such as the elderly. The cases that have spread beyond Mexico so far seem rather milder, but it is still too soon for a true picture of things to be clear.

What precautions can I take?

The good news is that antiviral drugs are known to be effective and can be used both for treatment and for prevention in appropriate circumstances. These are not vaccines, but come as capsules (Tamiflu) or an inhaler (Relenza). There is no reason for people to take these at the moment, unless they actually have symptoms or are at special risk of exposure. If the new strain begins to circulate in the UK, first-line precautions include frequent hand washing, possibly face masks, and minimising contact with people who are unwell.

Is it worth getting a flu jab?

There are only limited vaccine stocks now available, so this may be quite difficult, and it is not known whether the current seasonal flu vaccine offers worthwhile cross protection. At least one of the US cases occurred in a person known to have received this year's vaccine, but we can't yet know if he had a milder illness as a result.

What are the early symptoms of swine flu?

Fever, malaise, respiratory difficulties, and all the symtoms of "normal" flu.

What should I do if I show any symptoms?

If you are unwell with symptoms of flu, report them to your GP or to NHS 24, (08454 24 24 24, charged at local rate) and stay at home.

Should I cancel my holiday to Mexico or the US?

Mathematical modelling - conducted at Imperial College by Prof Neil Ferguson, who is an advisor to the NHS - suggests that restricting travel would make no significant difference to the international spread of flu viruses; and if the virus continues to spread, the risk of infection will probably be the same in the UK as it is in New Zealand, the US, or the other countries that have so far received cases. At the moment, the government and health authorities are not advising anyone travelling to Mexico or the other countries that have had cases, to change their plans, but the situation will need to be monitored very closely over the next days and weeks.